Aluminum oxide (alumina) has various crystalline forms such as γ-, δ-, θ-, and α-forms. The α-alumina is widely used as a raw material for fine ceramics in most general use. The γ-, δ-, and θ-aluminas other than the α-alumina are low-temperature phases of the α-alumina. They are referred to as transition alumina and used for catalyst carriers, various kinds of fillers, and modifiers for modifying Theological properties.
To produce the transition alumina, there is a process of calcining aluminum hydroxide, aluminum alkoxide, or alum, followed by grinding. Calcination of aluminum hydroxide, aluminum alkoxide, and alum can remove water, alcohol, and ammonium sulfate respectively therefrom, thereby providing alumina. In the above process, the particle properties of the raw material and the calcining conditions strongly influence the primary particle diameter and the secondary particle diameter of alumina obtained as the resulting product. Therefore, in order to control the primary and secondary particle diameters, it is important to pay attention to the properties of the raw particles, specifically, to appropriately select the raw materials. The step of grinding the calcined powder often becomes indispensable for regulating the particle size.
Further, as examples of the process for obtaining the transition alumina by synthesizing a raw material in a liquid phase and calcining the raw material, the following processes are known: a process for obtaining γ-alumina by preparing basic aluminum ammonium carbonate in a liquid phase and calcining the basic aluminum ammonium carbonate (Japanese Laid-Open Patent Application No. 11-228132); a process of gelling a basic aluminum chloride aqueous solution by pH adjustment and sintering the resulting gelled product (Japanese Laid-Open Patent Application No. 11-228131); and a process of sintering and grinding hydrated alumina with a boehmite structure (Japanese Laid-Open Patent Application No. 11-268911).
In addition to the above, there is a process for obtaining the transition alumina by a gas phase method, for example, a process for obtaining an ultrafine oxide by melting and vaporizing a metal in a vacuum vessel, and introducing the vaporized metal into an oxidizing atmosphere; a process of evaporating and oxidizing a metal aluminum powder in an oxygen-containing flame (Japanese Patent Publication No. 5-53722); and a process of ejecting a metal halide represented by anhydrous aluminum chloride into a burner flame to oxidize the same (Japanese Laid-Open Patent Application No. 8-197414). The transition alumina particles produced by such gas phase methods have a lower aggregation degree as compared with those of transition alumina produced through a liquid phase, or have a nearly spherical shape and are in almost a monodispersed state.
Recently, there has been a tendency for more and more increase in the degree of integration of circuits provided on a semiconductor substrate. The technique of chemical mechanical polishing (CMP) is attracting special attention as a method for increasing the degree of integration. In this technique, circuit formation on a substrate and smoothing of the substrate by polishing are alternately carried out to make a multi-layered circuit, thereby increasing the degree of integration. A slurry used for the CMP comprises an aqueous solvent of which the liquid properties are adjusted with an oxidizing metallic salt, a surfactant or the like, and inorganic particles uniformly suspended in the solvent. The aqueous solvent chemically reacts with the surface subjected to polishing, and the compounds present on the surface subjected to polishing are scraped away by the mechanical abrasiveness of the inorganic particles. Therefore, the inorganic particles are required to efficiently work to scrape away the compounds with a measure of hardness and to include neither coarse particles nor particles with extremely high hardness, as both types of particles would cause scratches on the surface subjected to polishing.
The transition alumina prepared through the sintering step includes a large number of coarse particles. It is difficult to decrease the number of coarse particles in order to cope with the CMP even if grinding is insistently conducted. Further, variance in sintering in the particles is unavoidable, and it is therefore highly probable that particles with high hardness are mixed in. Meanwhile, when the particles produced by the gas phase method are spherical or nearly spherical, the frictional force of the particles against the surface subjected to polishing becomes low and, consequently, the abrasiveness is decreased. In order to improve the abrasiveness, it is better for the primary particles or secondary particles to be large. However, scattering in the particle size distribution is unavoidable in light of the properties of the particles, so that no particles with a completely uniform particle size exist. On this account, with the increase in size of the primary particles or the secondary particles, the probability that coarse particles are contained is drastically increased, with the result that the occurrence of scratches becomes frequent. The above-mentioned Japanese Laid-Open Patent Application No. 8-197414 discloses fumed alumina produced by a gas phase method. However, the previously mentioned points are not taken into consideration, and the production process is not disclosed.